PRO rdmeandk,cam,exp,img,nointeractive=nointeractive,debug=debug
;
; Given camera number and exposure time (1, 10, 60, 120, 180 s
; only!), this procedure returns the appropriate mean dark image.
; Otherwise, it reads in the appropriate zero-time count and rate
; images and generates a synthetic dark frame for the requested
; exposure time.  Let's see how it works!
;

; Initialize OS-specific items
@isitdos

IF (exp EQ 1) OR (exp EQ 10) OR (exp EQ 60) OR (exp EQ 120) OR $
   (exp EQ 180) THEN BEGIN
    stexp=STRTRIM(FIX(exp),2)
    dkname=STRING(dkroot,dd,cc,cam,dd,cam,dc,exp,meantag $
                 ,FORMAT='(A,2A1,I1,A1,"dk",I1,A1,I3.3,"s",A)')
    df=FINDFILE(dkname,COUNT=nd)
    IF nd GT 1 THEN BEGIN
	IF NOT KEYWORD_SET(nointeractive) THEN BEGIN
            FOR j=0,nd-1 DO PRINT,j,df(j),FORMAT='(I1,2X,A)'
            PRINT,'Enter the number of the correct file'
            READ,ndc
	    ENDIF ELSE ndc=0
        dkname=df(ndc)
        ENDIF ELSE dkname=df(0)

    OPENR,dkunit,dkname,/GET_LUN
    fs=FSTAT(dkunit)
    IF fs.SIZE EQ 65536 THEN img=BYTARR(256,256) $
                        ELSE img=INTARR(256,256)
    READU,dkunit,img
    CLOSE,dkunit
    FREE_LUN,dkunit
    ENDIF ELSE BEGIN
    PRINT,STRING(STRTRIM(exp,2),cam $
         ,FORMAT='("Synthesizing ",A,"-s dark frame for camera ",I1)')

;   first read in the zero-time count image
    template=STRING(dkroot,dd,cc,cam,dd,cam,biastag $
            ,FORMAT='(A,A1,A,I1,A1,"dk",I1,A)')
    biasname=FINDFILE(template,count=nb)
    bias=BYTARR(256,256)
    IF nb GT 0 THEN BEGIN
        OPENR,dkunit,biasname(0),/GET_LUN
        READU,dkunit,bias
        CLOSE,dkunit
        FREE_LUN,dkunit
        ENDIF ELSE PRINT,'Can''t find the bias file!  Faking it...'

;   next read in the count rate image (DN/sec)
    template=STRING(dkroot,dd,cc,cam,dd,cam,ratetag $
            ,FORMAT='(A,A1,A,I1,A1,"dk",I1,A)')
    ratename=FINDFILE(template,count=nr)
    rate=BYTARR(256,256)
    IF nr GT 0 THEN BEGIN
        OPENR,dkunit,ratename(0),/GET_LUN
        READU,dkunit,rate
        CLOSE,dkunit
        FREE_LUN,dkunit
        ENDIF ELSE PRINT,'Can''t find the rate file!  Faking it...'

;   then synthesize the dark frame for the requested exposure time
    img=bias+exp*FLOAT(rate)/200.
    IF MAX(img) LE 255. THEN img=BYTE((img+0.5)>0) $
                        ELSE img=FIX((img+0.5)>0)
;   show what the synthetic image looks like
;   WINDOW,2,TITLE='Synthetic '+STRTRIM(exp,2)+'-s dark frame' $
;         ,XSIZE=256,YSIZE=256
;   TVSCL,img<255
;   WAIT,5
;   WDELETE,2
    ENDELSE

IF KEYWORD_SET(debug) THEN STOP

RETURN
END